The Indianapolis journal. [volume] (Indianapolis [Ind.]) 1867-1904, 1887-03-18
LETTERS FROM THE PEOPLE,.
Recent Researches in Terrestrial Magn etisnl.
To the Editor Of the Indianapulie Journal

Recent Researches in Terrestrial Magnetism
To the Editor Of the Indianapolis Journal.
Among the publications of the coast and
geodetc survey, appendix No of the last re-
port completing the investigation of terrestrial
magnetism, up to the present, is Of great i'm
portance. The magnetic needle is known not
to point to the geographical north pole. but to
vary from it by an angle, called the angle OF
defamation, or variation. This angle differs.
not only with the locality. but also with time.
In 1880 the needle pointed due north all
places situated on a line drawn from near the
western END of Lake Erie, thence coming south
eastwardly to the gulf. At every place east of
this line the needle pointed west of north, while
west of the line the declination was east In
1880 the needle at Indianapolis pointed @ AZ
east of north. This Zine of no variation" APP
pears to be moving westward. consequently west
of this line the needle is swinging nearer the
north and south position, while east of it the
westerly declination is increasing Now, 1887,
the needle here points only p 81 east of north,
and about 1900 its direction will coincide with
| that of the geographical meridian. After 1900
the declination here will be toward the
west. and will increase for about
$50 years, when. after short station
airy period, it will again slowly swing back to the
true north. This movement is apparently
caused by slow pendulumlike movement of
hundred years to complete a vibration from east
to west and return. Besides this slow move
| ment, the needle swings eastwardly every morn
ing and TO the west every evening. 1t also has
movement dependent the moon and vibra-
ton which it completes every year These facts
were discussed in the report for 1880 and are
cited to show how complex the magnetic phe-
nomena really are. The present paper treats of
the dip of the needle and its changes.
If steel needle is accurately balanced then
magnetized it will no longer remain horizontal.
but in the northern hemisphere the north end
will dip below the horizon AT Indianapolis the
dip or anglo made with the horizon is 70o 81. 4.
The force which directs the needle has been
accurately measured. For purposes of discus
sion it is divided into two portions one acting in
horizontal line. hence directing the needle
to the north. aud the other vertically, caus-
ing the dip. The first part of the
| report is occupied by tables of the
| dip at over two thousand places in the United
States. In the same tables are given the
amounts of the force in each locality. and the
amount supposed to act in the horizontal as
we'll that in the vertical, direction The
| material for these tables has been gathered from
every available source. much of it from work
done by observers sent out by the survey.
They serve as a basis for part second, which
consists of discussion of the change of the
| dip, which is known to be occurring The fact
| that the dip varied was first discussed, in Amer
| lea. at least, by Prof. Looinis in 1888 Subse-
quent study shows that the dip decreased, or
the needle became more nearly horizontal until
1913, when it began to increase. This increase
| continued until about 1859, when annual
| decrease began, and has since continued. 1t is
| evident that these changes must follow some
| law, but what that law is our knowledge is not
complete enough to determine. There maps
| added which show lines connecting the places at
| which the dip is equal. and those at which the
| force is the same. 1t appears that in the United
| States the dip is decreasing, while at the same
| time in Mexico, southern Texas and southwark
| 1t is increasing. j,